For example, an inspecting target object is a metal member (steel member) including a welded part, and a position of a defect in the welded part of the inspecting target object is acquired by an ultrasonic wave.
As this inspection method, there is a time-of-flight diffraction (TOFD) method.
This TOFD method is performed as follows. In FIG. 1, an ultrasonic transmitter 33 generates an ultrasonic wave. This ultrasonic wave includes a lateral wave propagating on a surface of an inspecting target object 1, and an inside-propagation wave propagating inside the inspecting target object 1. The inside-propagation wave includes a wave reflected by an upper end of a defect 1a (defect in a welded part in this example) inside the inspecting target object 1, a wave reflected by a lower end of the defect 1a, and a wave reflected by a bottom of the inspecting target object 1. A timing of generating the ultrasonic wave is set as an origin for time measurement. The time that an ultrasonic receiver 35 detects the lateral wave is set as t1. The time that the ultrasonic receiver 35 detects the reflection wave from the upper end of the defect 1a is set as t2. The time that the ultrasonic receiver 35 detects the reflection wave from the lower end of the defect 1a is set as t3. The time that the ultrasonic receiver 35 detects the reflection wave from the bottom of the inspecting target object 1 is set as t4. While a distance between the ultrasonic transmitter 33 and the ultrasonic receiver 35 is kept constant, an attached position of the ultrasonic transmitter 33 and the ultrasonic receiver 35 to the inspecting target object 1 is gradually moved to the right side in FIG. 1, and at each attached position, the above-mentioned time t1 to t4 is detected by the ultrasonic transmitter 33 and the ultrasonic receiver 35.
The thus-acquired time data is illustrated in FIG. 2 (illustration of t4 is omitted). In FIG. 2, the horizontal axis indicates a position of the ultrasonic transmitter 33, and the vertical axis indicates time in which the timing of generating the ultrasonic wave is the origin. The time t1 to t3 is expressed by the straight line or the curved lines illustrated in FIG. 2. It can be assumed that when the time t2 and the time t3 becomes minimum, a distance from the ultrasonic transmitter 33 to the defect 1a is equal to a distance from the ultrasonic receiver 35 to the defect 1a. Accordingly, on the basis of the minimum values of t2 and t3, and a known distance between the ultrasonic transmitter 33 and the ultrasonic receiver 35, a position (a location in the left-to-right direction and a distance from this location to the defect 1a in FIG. 1) of the defect 1a can be acquired.
Such a TOFD method is described in below-mentioned Patent Literature (PTL) 1, for example. Concerning amplitude modulation in an embodiment of the present invention, there is below-mentioned Non-Patent Literature (NPL) 1.